Recently, redox catalysts in which metal is supported on the surface of a carbonaceous catalyst carrier have been proposed for use for example in electrodes for fuel cells. It has also been proposed to use carbon nanotubes as a catalyst carrier for such redox catalysts. Metal-supporting carbon nanotubes are generally prepared by attaching a metal precursor onto the surface of carbon nanotubes, reducing the metal precursor into metal, and fixing the metal by sintering. This preparation method, however, has a problem of unwanted aggregation of carbon nanotubes during sintering.
On the other hand, exposure of metal on the surface of carbon nanotubes results in a problem of elution of the metal by contact with electrolyte for example when the metal-supporting carbon nanotubes are used in an electrode of a fuel cell as a redox catalyst. Known techniques for solving this problem involve covering metal-supporting carbon nanotubes with a protection film made of inorganic material such as silica. Nevertheless, these techniques suffer from a problem of inability to well cover metal-supporting carbon nanotubes with a protection film made of inorganic material when the carbon nanotubes are aggregated as described above.
To solve this problem, PTL 1 discloses a technique which involves attaching a metal precursor to the surface of carbon nanotubes, reducing the metal precursor to its reduced state, and further covering the metal-supporting carbon nanotubes with a protection film made of inorganic material, followed by fixation of the metal by sintering. In this case, since metal fixation by sintering is not performed prior to coverage by the protection film, aggregation of the carbon nanotubes does not take place, allowing the metal-supporting carbon nanotubes to be sufficiently covered by the protection film.